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Full text of "Flora Indica Vol-I"

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INTRODUCTORY  ESSAY.                                    75
grand elements -which give the character to the climate, react
naturally upon one another, so that it is not easy to deter-
mine which is the cause and which the effect.
For all practical purposes we may regard the sun as the
sole source of the temperature of the surface of the globe.
If the surface of our planet were uniform, the sun's heating
power would be directly proportional to his altitude, and the
mean temperature would diminish equably in receding from
the equator. A variety of circumstances disturb this regular
gradation of temperature. These are—1. The more rapid
heating and cooling of land than sea, which arises in a great
measure from the heat being gradually diffused throughout
the ocean (by means of oceanic currents), the hot water from
the tropics being thus carried into temperate regions, while
tlic cold water of the Arctic seas occupies its place. Proxi-
mity to the ocean, therefore, promotes uniformity of tem-
perature.—2. The, elevation of the land above the level of
tlic sea. The sun's heating power is ratlier augmented at
great elevations; but a diminution of temperature at high
levels is caused by the rarefaction of the air, and is a conse-
quence of the law according to which, the specific heat of the
atmosphere increasing inversely with its density, its setisibh*
heat becomes absorbed as it expands. As this law is universal,
it follows, that when a current of air ascends or descends, its
temperature is changed to an amount exactly proportional to
the change of level; and it is only wheu such a current is
hotter thau the normal temperature of the place whence it
ascends, that it is a warm "wind at a higLcr level.—3. The
presence or absence of clouds. These intercept the solar rays
(faring the day, and tend to keep the ground riooL During
thr* insist, OH the -contrary, cioud» intercept the*, tftdiatiou of
tin* liral accumulated in the earth during the day, and tend
to ki*{*,p -the ground warm* A cloudy climate is hetfce an
equable one, having comparatively cool days anil warm nights,
cool summers and niiM winters.
When the sky *s nloar, tlie air iu comurt with the earth
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